1. Field of the Invention
The present invention is directed to low pressure mercury vapor lamps, more commonly known as fluorescent lamps, having a lamp envelope with phosphor coating, and more particularly, to a coating with four phosphors over an alumina pre-coat.
2. Discussion of the Prior Art
Low pressure mercury vapor lamps, more commonly known as fluorescent lamps, have a lamp envelope with a filling of mercury and rare gas to maintain a gas discharge during operation. The radiation emitted by the gas discharge is mostly in the ultraviolet (UV) region of the spectrum, with only a small portion in the visible spectrum. The inner surface of the lamp envelope has a luminescent coating, often a blend of phosphors, which emits visible light when impinged by the ultraviolet radiation. Special fluorescent lamps known as cool white deluxe (CWX) have high color rendering and simulate natural light. CWX lamps are used in places where it is desired to simulate natural light, such as in retail stores for clothing and furniture.
The phosphors of conventional CWX lamps are high mercury consumers and cannot pass the Toxicity Characteristic Leaching Procedure (TCLP) test without sacrificing lamp life. Accordingly, there is a drive to reduce mercury consumption in conventional CWX fluorescent lamps without a significant reduction in the lamp life or change in the color characteristics of the emitted light.
To increase efficiency and reduce mercury consumption without a significant reduction in the lamp life or change in the color characteristics of the emitted light, different blends of phosphors are used for the luminescent coating. Further, a metal oxide layer is provided between the luminescent coating and glass envelope. The metal oxide layer reflects the UV radiation back into the phosphor luminescent layer through which it has already passed for further conversion of the UV radiation to visible light. This improves phosphor utilization and enhances light output. The metal oxide layer also reduces mercury consumption by reducing mercury bound at the tubular portion of the lamp.
Desirable fluorescent lamps characteristics include high brightness and high color rendering. Conventional CWX lamps have a correlated color temperature of approximately 4100 K, with a color rendering indices (CRI) greater than 88. In particular, conventional CWX lamps are made with a two-phosphor mixture of Strontium Magnesium Orthophosphate (St. Mag), i.e., (Sr,Mg)3(PO4)2:Sn, and Strontium Halophosphate (St. Blue), i.e., Sr10(PO4)6F2:Sb. The St. Mag is very rich in the red region of the spectrum and the St. Blue provides the conventional CWX lamp with the blue light source.
These phosphors are detrimental for mercury consumption. In particular, St. Mag is the highest consumer of mercury and its high percentage renders the conventional CWX lamps non-TCLP compliant.
Accordingly, there is a need for fluorescent CWX lamps with reduced mercury that pass TCLP without affecting characteristics thereof, such as maintaining a high CRI of greater than 88 and substantially the same correlated color temperature (CCT) or color point coordinates.
The object of the present invention is to provide fluorescent CWX lamps with high CRI and reduced mercury consumption.
The present invention accomplishes the above and other objects by providing an electric lamp having an envelope with an inner surface and at least one electrode, such as two electrodes located at both ends of the envelope tube. The electrodes transfer electric power to generate ultraviolet radiation in the envelope which is filled with mercury and a charge sustaining gas. The inner surface of the envelope is pre-coated with a metal oxide layer, such as an aluminum oxide layer, to reflect ultraviolet radiation back into the envelope.
A phosphor layer is formed over the aluminum oxide to convert the ultraviolet radiation to visible light. The phosphor layer is a mixture of four phosphors, namely, Blue Halophosphate (BH), i.e., Ca10(PO4)6F2:Sb, red-luminescing Yittrium Oxide (YOX), i.e., Y2O3:Eu, 2900K-luminescing Calcium Halophosphate, also referred to as Warm White Halophosphate (WW), i.e., Ca10(PO4)6(F,Cl)2:Sb,Mn, and green-luminescing Zinc Silicate (ZS), i.e., Zn2SiO4:Mn.